This invention relates to a method of addressing a ferroelectric liquid crystal device (FLCD), in particular to a method of controlling the transmission of electromagnetic radiation through such a device. This method is particularly, though not exclusively, intended for addressing such a device used as an optical shutter. It is envisaged that such a method could be used to control the transmission through a FLCD of electromagnetic radiation of other wavelengths e.g. infra-red and ultra-violet radiation as well as optical radiation.
Ferroelectric liquid crystal materials have a DC voltage response. An FLCD containing such a material between polarizers can be switched from a light transmissive state to a non-transmissive state and vice versa by an applied voltage of sufficient magnitude and pulse width, the state into which it is switched being dependent upon the polarity of the applied voltage. A variety of voltage waveforms can be used but a waveform with a step function, e.g. a square wave pulse, is preferred for a minimum rise and fall time (fast response). FIG. 1 shows an electro-optic characteristic, i.e. a plot of pulse height V.sub.S against pulse width t.sub.S of a monopolar pulse wave (see inset - FIG. 1) to produce switching from a light transmissive state to a non-transmissive state or vice versa for a layer of a typical ferroelectric liquid crystal material, such as SCE13 (supplied by BDH Ltd., Poole, UK). The layer was 1.5 .mu.m thick and the temperature was 25.degree. C.
FIG. 2 shows a graph of voltage applied to a ferroelectric liquid crystal layer against time and a graph of optical transmission of that liquid crystal layer over the same time. Monopolar pulses of sufficient pulse height V.sub.S and pulse width t.sub.S to switch the liquid crystal layer between a first state T.sub.X1 of maximum optical transmission and a second state T.sub.X2 of minimum optical transmission are applied. The ideal optical transmission curve is shown in dotted lines - the liquid crystal is latched in the first or second state until a pulse of the polarity required to switch it into the other state is applied. However, in a practical embodiment some relaxation of the latched states usually occurs within a period of 10t.sub.S and the separation of the monopolar pulses is greater than this. The continuous curve of FIG. 2 shows this relaxation which reduces the contrast ratio, an undesirable effect for a light shutter.
A variety of addressing schemes have been tried to avoid the problem of relaxation. In one scheme, as shown in FIG. 3, the device is switched between the first and second states T.sub.X1, T.sub.X2 by a continuously applied AC square wave voltage. The AC square wave voltage pulses are of sufficient height V.sub.S and pulse width t.sub.S to switch between the first and second states. The applied voltage V.sub.s prevents relaxation occurring and maintains the liquid crystal cell in the T.sub.x1 or T.sub.x2 state, ensuring that the contrast remains high. However the alignment of the liquid crystal layer in the device can easily be damaged in an irreversible manner when alternating electric fields above a critical value are applied. Alignment damage to the liquid crystal layer reduces the contrast ratio of the shutter and tends to increase the response time of the material. For many materials, the critical value is typically of the order of 10V/ .mu.m - well below that usually required to realize the maximum switching speed.
In an alternative scheme, as shown in FIG. 4, as high frequency background AC signal of voltage magnitude V.sub.AC is applied to stabilize the states T.sub.X1 and T.sub.X2. When V.sub.AC has a finite value V.sub.a, there is stabilization whereas when V.sub.AC 32 0, relaxation occurs. Unfortunately the value of the fields necessary for AC stabilization can depend on a variety of parameters such as cell thickness, preparation of the alignment layer material and physical properties of the liquid crystal material, such as its dielectric anisotropy e.g. as disclosed by T.Umeda et al : Influences of Alignment Materials and LC Layer Thickness on AC Field - Stabilization Phenomena of Ferroelectric Liquid Crystals (Japanese Journal of Applied Physics Vol. 27. No. 7. Jul. 1988, pages 1115-1121) and T. Nagata et al : Physical Properties of Ferroelectric Liquid Crystals and AC Stabilization Effect (Japanese Journal of Applied Physics Vol. 27. No. 7. Jul. 1988, pages 1122-1125). With many liquid crystal materials, AC stabilization is not very successful. Often large AC fields are required which are about or greater than the critical value which will produce alignment damage to the liquid crystal layer and reduce the contrast ratio.
GB 2175725A (Seikosha) discloses a method of driving an electro-optical display device (such as an FLCD) for producing a display consisting of display elements and which comprises first and second sets of electrodes, the electrodes of one set crossing those of the other. A selection signal is sequentially applied to the first set of electrodes while a non-selection signal is applied to each of the first set of electrodes to which the selection signal is not applied. In the methods described, defining a display element, the resultant waveform across that display element is a substantially true pulsed AC waveform. In two embodiments, this substantially having a reduced duration half or less than half of the duration of the switching pulse followed by two pulses of the same reduced duration but of the other polarity. The provision of a substantially time pulsed AC waveform ensures that the substantially transparent electrodes do not become blackened, the liquid crystal material does not deteriorate and double colour pigment does not become discoloured, even after driving for a long time. The AC waveform provided during non-selection also provides good contrast.
US 4508429 (Nagae et al) discloses a FLC display in which two light transmitting states, i.e. a bright state and a dark state, can be established. Each of these states is defined by the average brightness brought about by pulse voltage trains of a respective polarity. Each pulse in the pulse voltage trains shown is of the same pulse height which is accordingly sufficient to switch the FLC display from one defined light transmitting state to the other and vice versa. However, a problem with this driving method is that, unless the duration of the bright display state is equal to that of the dark display state, the voltage V.sub.LC applied to the FLC will include a DC component. US 4508429 discloses that ` It is well known that when a DC component is applied to a liquid crystal element during the driving thereof, the deterioration of the element is accelerated because of an electrochemical reaction, thereby resulting in a reduced life.`